Investigation of Geothermal Well Cement Integrity in the Presence of Fibre Optic Cable Installation: A Numerical and Experimental Approach

Cement integrity is a critical aspect of ensuring a reliable and sustainable life for a geothermal well, where compromising factors such as cycling thermal stress, corrosive formation fluids, and the formation's nature put the well integrity under a long-term test. The integration of fibre optic cable adds a significant advantage to monitoring the well in real-time; however, the installation within the cement sheath can locally disturb the microstructure, alter stress distribution, and potentially initiate microcracks and/or micro-annuli around the cable that threaten overall well integrity.
This work is aiming to evaluate whether the cement sheath integrity could be compromised by the presence of the fibre optic cable or not. And hence it investigates the coupled mechanical behaviour of geothermal well cement in the presence of embedded fibre optic cables through a combined experimental and numerical approaches including uniaxial compressive strength (UCS), N2 permeability, optical & digital microscopy, ultrasonic wave readings, indirect tensile strength test, post cutting posh-out bond test, and the FEA simulations as well as some mechanical calculations.

As to the methodology, after a successful preliminary manual bond check with 1-inch specimen, a simulation of strength tests was conducted to get the minimum specimen size for UCS tests with cable. This was found to be 2-inch diameter samples above which the perturbations caused by the installation of the cable can be ignored for the laboratory strength test purpose. 32 specimens, as well as two casing-cement-cable assemblies, were prepared corresponding to the two cement recipes, as well as destructive and non-destructive tests that were planned. An important point is that the idea of this thesis was quite novel, hence the author had to develop new approaches addressing the faced challenges. Cases in point were the optimum specimen size design, cutting the samples, casing with the least damage to the casing-cable-cement bond, and developing the FEA codes to simulate the UCS tests.

The results show that the presence of a permanently embedded fibre-optic cable in the cemented wellbore annulus does not compromise the bulk mechanical integrity of geothermal well cement when proper slurry formulation and curing conditions are applied. Destructive and non-destructive strength tests indicate that cement strength and stiffness are primarily governed by curing quality, with the Tail cement consistently outperforming the Lead cement. Ultrasonic pulse velocity measurements confirm the strength trend of Tail > Tail + Cable > Lead > Lead + Cable, while showing only minor reductions due to cable presence. In contrast, post-cutting investigations reveal that interface behaviour is highly sensitive to mechanically introduced damage: cutting and polishing induce significant stresses that cause partial or complete cement–cable debonding, particularly in the Lead cement. Despite this, the push-out tests (still post cutting) show that the Tail cement holds a residual cable–cement interaction nearly four times stronger than that of the Lead cement, even after bond degradation. Overall, the findings indicate that long-term integrity risks are governed by interface mechanics, thermal mismatch, and curing conditions rather than by inherent degradation of the cement matrix due to fibre-optic cable installation.